Here are a couple ways:
1. You could turn off the the display of those lines (i.e the contours themselves), and then plot a circle around the perimeter to help differentiate the lighter areas from the background:
set(get(gca,'Children'),'LineStyle','none')
hc=circle(0,0,1,72);
set(hc,'Color','k')
Here I used the following simple function to draw the circle (I think I got it from the FEX ages ago):
function h = circle(x,y,r,nsegments)
if nargin<4
nsegments=50;
end
hold on
th = 0:2*pi/nsegments:2*pi;
xunit = r * cos(th) + x;
yunit = r * sin(th) + y;
h = plot(xunit, yunit);
hold off
end
2. You could just deemphasize the lines by making them the dotted style:
set(get(gca,'Children'),'LineStyle',':')
